Contactless nonvolatile semiconductor memory device having buried bit lines surrounded by grooved insulators

ABSTRACT

In a contactless nonvolatile semiconductor memory device including a semiconductor substrate and a plurality of impurity diffusion layers of a rectangular shape serving as sub bit lines on the semiconductor substrate, a plurality of grooves of a rectangular shape are formed in the semiconductor substrate between the impurity diffusion layers. Also, a first gate insulating layer is formed on the semiconductor substrate within the grooves, and a plurality of floating gate electrodes are formed on the gate insulating layer. Further, a second gate insulating layer is formed on the floating gate electrodes, and a plurality of word lines are formed on the second gate insulating layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a contactless nonvolatile semiconductor memory device.

2. Description of the Related Art

In a nonvolatile semiconductor memory device such as a flash electrically-erasable programmable read-only memory (EEPROM) or an electrically programmable read-only memory (EEPROM), the integration has been advanced by constructing bit lines as buried impurity diffusion layers.

In a prior art contactless nonvolatile semiconductor memory device, buried impurity diffusion layers as bit lines are formed beneath thick insulating layers (see J. Esquivel et al., "High Density Contactless, Self-aligned EEPROM Cell Array Technology", IEDM Technical Digest, pp. 592-595, 1986). This will be explained later in detail.

In the above-mentioned prior art contactless nonvolatile semiconductor memory device, however, when the integration is advanced so that a spacing between the buried impurity diffusion layers becomes narrow, leakage currents flowing therebetween are increased, which invites a malfunction of nonvolatile memory cells. Also, this creates a serious short channel effect.

In addition, when patterning floating gate electrodes by a dry etching process, the buried impurity diffusion layers are also etched. As a result, the resistance of the buried impurity diffusion layers is increased, which reduces the read operation speed of the device.

SUMMARY OF THE INVENTION

It is an object of the present invention to reduce the leakage currents between buried impurity diffusion layers serving as bit lines in a contactless semiconductor memory device.

Another object of the present invention is to suppress the short channel effect of memory cells.

A further object of the present invention is to avoid the etching of the buried impurity diffusion layers.

According to the present invention, in a contactless nonvolatile semiconductor memory device including a semiconductor substrate and a plurality of impurity diffusion layers serving as sub bit lines of a rectangular shape on the semiconductor substrate, a plurality of grooves of a rectangular shape are formed in the semiconductor substrate between the impurity diffusion layers. Also, a first gate insulating layer is formed on the semiconductor substrate within the grooves, and a plurality of floating gale electrodes are formed on the first gate insulating layer. Further, a second gate insulating layer is formed on the floating gate electrodes, and a plurality of word lines are formed on the second gate insulating layer. Thus, the distance between the buried impurity diffusion layers is increased by the presence of the grooves, which reduces leakage currents therebetween.

Also, a plurality of insulating layers are formed on the buried impurity diffusion layers. Thus, when patterning floating gate electrodes, the buried impurity diffusion layers are hardly etched due to the presence of the insulating layers thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood from the description as set forth below, in comparison with the prior art, with reference to the accompanying drawings, wherein:

FIG. 1 is an equivalent circuit diagram illustrating a prior art contactless nonvolatile semiconductor memory device;

FIG. 2 is a plan view of the device of FIG. 1;

FIGS. 3A and 3B are cross-sectional views taken along the lines I--I and II--II, respectively, of FIG. 1;

FIG. 4 is a plan view illustrating a first embodiment of the contactless nonvolatile semiconductor memory device according to the present invention;

FIGS. 5A, 5B, 6A, 6B, 7A, 7B, 8A, 8B, 9A, 9B, 10A, 10B, 11A, 11B, 12A, 12B, 13A, 13B, 14A, 14B, 15A, 15B, 16A, 16B, 17A and 17B are cross-sectional views showing a first method for manufacturing the device of FIG. 4;

FIGS. 18A, 18B, 19A, 19B, 20A, 20B, 21A, 21B, 22A, 22B, 23A, 23B, 24A, 24B, 25A, 25B, 26A, 26B, 27A, 27B, 28A, 28B, 29A, 29B, 30A, 30B, 31A and 31B are cross-sectional views showing a second method for manufacturing the device of FIG. 4;

FIG. 32 is an equivalent circuit diagram illustrating another prior art contactless nonvolatile semiconductor memory device;

FIG. 33 is a plan view illustrating a second embodiment of the contactless nonvolatile semiconductor memory device according to the present invention;

FIGS. 34A and 34B are cross-sectional views taken along the lines I--I and II--II, respectively, of FIG. 33;

FIG. 35 is a cross-sectional view showing a method for manufacturing the device of FIG. 33;

FIG. 36 is a plan view illustrating a third embodiment of the contactless nonvolatile semiconductor memory device according to the present invention; and

FIGS. 37A, 37B, 38A, 38B, 39A, 39B, 40A, 40B, 41A, 41B, 42A, 42B, 43A, 43B, 44A, 44B, 45A, 45B, 46A, 46B, 47A, 47B, 48A, 48B, 49A, 49B, 50A, 50B, 51A, 51B, 52A, 52B, 53A and 53B are cross-sectional views showing a method for manufacturing the device of FIG. 36.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before the description of the preferred embodiments, a prior art contact-Less nonvolatile semiconductor memory device will be explained with reference to FIGS. 1, 2, 3A and 3B (see J. Esquivel et al., "High Density Contactless, Self-aligned EPROM Cell Array Technology", IEDM Technical Digest, pp. 592-595, 1986).

In FIG. 1, which is an equivalent circuit diagram illustrating a prior art contactless virtual ground type nonvolatile semiconductor memory device, references WL₁, WL₂, . . . designate word lines, MBL₁, MBL₂, . . . designate main bit line lines; SBL₁, SBL₁, . . . designate sub bit lines arranged along with the main bit lines MBL₁, MBL₂, . . . , respectively; and M₁₁, M₁₂, M₁₃, . . . , M₂₁, M₂₂, M₂₃, . . . designate floating gate type nonvolatile memory cells each connected to one of the word lines WL₁, WL₂, . . . and one of the sub bit lines SBL₁, SBL₂, . . . The main bit lines MBL₁, MBL₂, . . . are formed by conductive layers, while the sub bit lines SBL₁, SBL₂, . . . are formed by buried impurity diffusion regions in a semiconductor substrate. The sub bit lines SBL₁, SBL₂, . . . are connected to the main bit lines MBL₁, MBL₂, . . . , respectively, via switching transistors Q₁, Q₂, . . . , Q₁ ', Q₂ ', . . . which are turned ON and OFF by a block selection signal BSEL₁.

The nonvolatile memory cells M₁₁, M₁₂, M₂₁ and M₂₂ of FIG. 1 are explained next with reference to FIGS. 2, 3A and 3B. Here, FIG. 2 is a plan view of the nonvolatile memory cells M₁₁, M₁₂, M₁₃, M₂₁, M₂₂ and M₂₂ of FIG. 1, and FIGS. 3A and 3B are cross-sectional views taken along the lines I--I and II--II, respectively, of FIG. 2.

That is, N⁺ -type impurity diffusion regions 102 are formed (as the sub bit lines SBL₂ and SBL₃) within a P⁻ -type silicon substrate 101. Also, a thick silicon oxide layer 103 is formed on the N⁺ -type impurity diffusion regions 102. Further, a gate silicon oxide layer 104 and a conductive layer 105 serving as floating gate electrodes FG are formed on the silicon substrate 101. In addition, a gate silicon oxide layer 106 and a conductive layer 107 serving as word lines WL₁, WL₂ as well as control gates CG are formed. Further, P-type channel stopper regions 108 are formed within the silicon substrate 101 between the floating gate electrodes FG. Finally, an insulating layer 109 and a conductive layer 110 serving as the main bit lines MBL₂ and MBL₃ are formed.

In the prior art contactless nonvolatile semiconductor memory device of FIGS, 1, 2, 3A and 3B, however, when the integration is advanced so that a spacing between the N⁺ -type impurity diffusion regions 102 (SBL₂, SBL₃) becomes narrow, leakage currents flowing therebetween are increased, which invites a malfunction of the nonvolatile memory cells M₁₁, M₁₂, . . . , Also, this creates a serious short channel effect.

In addition, when the conductive layer 107 (WL₁, WL₂, CG) is patterned by a dry etching process, the conductive layer 105 (EG) is also patterned in self-alignment with the conductive layer 107. In this case, since the thickness of the gate silicon oxide layer 104 on the N⁺ -type impurity diffusion regions 102 (SBL₂, SBL₃) is insufficient, the N⁺ -type impurity diffusion of regions 102 (SBL₂, SBL₃) are partly etched by the above-mentioned dry etching process. As a result, the resistance of the N⁺ -type impurity diffusion regions 102 (SBL₂, SBL₃) is increased, which reduces the read operation speed of the device.

In FIG. 4, which is a plan view illustrating a first embodiment of the present invention, a contactless virtual ground type nonvolatile semiconductor device whose equivalent circuit is also illustrated in FIG. 1 Grooves GV are provided between sub bit lines SBL₁, SBL₂, SBL₃, . . . which are also formed by buried impurity layers. As a result, a distance between the sub bit lines SBL₁, SBL₂, SBL₃, . . . is increased, substantially and thus, leakage currents therebetween can be decreased, even if an erase voltage is increased. In addition, since insulating layers (not shown) are formed completely on the sub bit lines SBL₁, SBL₂, SBL₃, . . . , when patterning floating gate electrodes FG by a dry etching process, the sub bit lines SBL₁, SBL₂, SBL₃, . . . are hardly etched.

In FIG. 4, note that references WL₁, WL₂, . . . designates word lines, and STP designates a channel stopper.

A first method for manufacturing the contactless virtual ground type nonvolatile semiconductor memory device of FIG. 4 is explained next with reference to FIGS. 5A, 5B, 6A, 6B, 7A, 7B, 8A, 8B, 9A, 9B, 10A, 10B, 11A, 11B, 12A, 12B, 13A, 13b, 14A, 14B, 15A, 15B, 16A, 16B, 17A and 17B. Note that FIGS. 5A through 17A are cross-sectional views taken along the line I--I of FIG. 4, and FIGS. 5B through 17B are cross-sectional views taken along the line II--II of FIG. 4.

First, referring ot FIGS. 5A and 5B, an about 10 to 20 mn thick silicon oxide layer 2 is formed on a P⁻ -type silicon substrate 1 by using a thermal oxidation process or a chemical vapor deposition process (CVD) process. Then, about 1×10¹⁵ to 7×10¹⁵ arsenic ions/cm³ are implanted at an energy of about 50 kev into the silicon substrate 1, and an annealing operation is performed thereupon. Thus, an N⁺ -type impurity diffusion layer 3 is formed.

Next, referring to FIGS. 6A and 6B, the silicon oxide layer 2 is removed.

Next, referring to FIGS. 7A nad 7B, an about 200 to 500 nm thick silicon oxide layer 4 is deposited on the N⁺ -type impurity diffusion layer 3.

Next, referring to FIGS. 8A and 8B, the silicon oxide layer 4 and the N⁺ -type impurity diffusion layer 3 are patterned by a photolithography and reactive ion etching (RIE) process. As a result, patterned N⁺ -type impurity diffusion layers 3a are in self-alignment with patterned silicon oxide layers 4a. In this case, the N⁺ -type impurity diffusion layers 3a are of a rectangular shape and form the sub bit lines SBL₁, SBL₂, and SBL₃ of FIG. 4.

Subsequently, referring to FIGS. 9A and 9B, the silicon substrate 1 is further etched by the above-mentioned RIE process using the silicon oxide layers 4a as a mask. As a result, grooves 5 of a rectangular shape viewed from the top, i.e., the grooves GV of FIG. 4 are formed in self-alignment with the silicon oxide layers 4a,

Next, referring to FIGS. 10A and 10B, a thermal oxidation operation is carried out, so that an about 7 to 10 nm thick gate silicon oxide layer 6 is formed.

Next, referring to FIGS. 11A and 11B, a polycrystalline silicon layer 7 including phosphorus is deposited by a CVD process on the entire surface.

Next, referring to FIGS. 12A and 12B, the polycrystalline silicon layer 7 is patterned by a photolithography and etching process, so that polycrystalline silicon layers 7a of a rectangular shape viewed from the top are formed.

Next, referring to FIGS. 13A and 13B, a gate insulating layer 9 made of silicon oxide/silicon nitride/silicon oxide (ONO), is formed on the entire surface, and a polycrystalline silicon layer 10 including phosphorus is deposited on the gate insulating layer 9 by a CVD process.

Next, referring to FIGS. 14A and 14B, the polycrystalline silicon layer 10 is patterned by a photolithgraphy and dry etching process. As a result, patterned polycrystalline silicon layers 10a, i.e., the word lines WL₁ and WL₂ of FIG. 4 are obtained.

Next, referring to FIGS. 15A and 15B, the polycrystalline silicon layers 7a are further etched by a dry etching process by using the polycrystalline silicon layers 10a (the word lines WL₁) as a mask. As a result, patterned polycrystalline silicon layers 7b, i.e., the floating gate electrodes FG of FIG. 4 are obtained. Then, about 1×10¹³ boron ions/cm² are implanted at an energy of about 30 keV by using the polycrystalline silicon layers 10a (WL₁, WL₂) and the silicon oxide layers 4a as a mask. Thereafter, an annealing operation is carried out. As a result, P-type channel stopper layers 8, i.e., the channel stopper layers STP of FIG. 4 are formed.

Next, referring to FIGS. 16A and 16B, a thick silicon oxide later 11 is deposited by a CVD process, and then, a chemical mechanical polishing (CMP) operation is performed on the silicon oxide layer 11. Thus, the surface of the silicon oxide layer 11 is flattened. Then, an aluminum layer 12 is deposited on the silicon oxide layer 11 by a sputtering process.

Finally, referring to FIGS. 17A and 17B, the aluminum layer 12 is patterned by a photolithography and etching process, so that patterned aluminum layers 12a, i.e., the main bit lines MBL₁, MBL₂ and MBL₃ of FIG. 4 are obtained.

Thus, the contactless virtual ground type nonvolatile semiconductor device of FIG. 4 is completed.

A second method for manufacturing the contactless virtual ground type nonvolatile semiconductor memory device of FIG. 4 is explained next with reference to FIGS. 18A, 18B, 19A, 19B, 20A, 20B, 21A, 21B, 22A, 22B, 23A, 23B, 24A, 24B, 25A, 25B, 26A, 26b, 27A, 27B, 28A, 28B, 29A, 29B, 30A, 30B, 31A and 31B. Note that FIGS. 18A through 31A are cross-sectional views taken along the line I--I of FIG. 4, and FIGS. 18B through 31B are cross-sectional views taken along the line II--II of FIG. 4.

First, referring to FIGS. 18A and 18B, a silicon oxide layer 22 is grown on a silicon substrate 21.

Next, referring to FIGS. 19A and 19B, the silicon oxide layer 22 is patterned by a photolithography and etching process, so that patterned silicon oxide layers 22a are formed.

Next, referring to FIGS. 20A and 20B, the silicon substrate 21 is etched by an RIE process using the silicon oxide layers 22a as a mask. As a result, grooves 23 of a rectangular shape viewed from the top, i.e., the grooves GV of FIG. 4 are formed in self-alignment with the silicon oxide layers 22a. Then, the silicon oxide layers 22a are removed.

Next, referring to FIGS. 21A and 21B, a thermal oxidation operation is carried out, so that an about 7 to 10 nm thick gate silicon oxide layer 24 is formed.

Next, referring to FIGS. 22A and 22B, a polycrystalline silicon layer 25 including phosphorus is deposited by a CVD process on the entire surface.

Next, referring to FIGS. 23A and 23B, the polycrystalline silicon layer 25 is patterned by a photolithography and etching process, so that polycrystalline silicon layers 25a of a rectangular shape are formed.

Next, referring to FIGS. 24A and 24B, about 1×10¹⁵ to 5×10¹⁵ arsenic ions/cm² are implanted at an energy of about 50 keV into the silicon substrate 21, by using the polycrystalline silicon layers 25a as a mask, and an annealing operation is performed thereupon. Thus, N⁺ -type impulity diffusion layers 26, i.e., the sub bit lines SBL₁, SBL₂ and SBL₃ of FIG. 4 are formed.

Next, referring to FIGS. 25A and 25B, silicon oxide layers 27 are formed between the polycrystalline silicon layers 25a. That is, a silicon oxide layer is deposited on the entire surface by a CVD process, and a CMP operation is performed upon the silicon oxide layer. As a result, the silicon oxide layers 27 are left only on the N⁺ -type impurity diffusion layers 26.

Next, referring to FIGS. 26A and 26B, a silicon oxide layer 29 is formed on the polycrystalline silicon layers 25a, and a polycrystalline silicon layer 10 including phosphorus is deposited on the entire surface by a CVD process.

Next, referring to FIGS. 27A and 27B, the polycrystalline silicon layer 30 is patterned by a photolithgraphy and dry etching process. As a result, patterned polycrystalline silicon layers 30a, i.e., the word lines WL₁ and WL₂ of FIG. 4 are obtained.

Next, referring to FIGS. 28A and 28B, the polycrystalline silicon layers 25a of a rectangular shape are further patterned by a photolithography and etching process using the word lines WL₁, WL₂, . . . as a mask, so that patterned polycrystalline silicon layers 25b, i.e., the floating gate electrodes FG of FIG. 4 are formed.

Next, referring to FIGS. 29A and 29B, about 1×10¹³ boron ions/cm² are implanted at an energy of about 30 keV by using the polycrystalline silicon layers 25a (FG) and the silicon oxide layers 27 as a mask. Thereafter, an annealing operation is carried out. As a result, P-type channel stopper layers 28, i.e., the channel stopper layers STP of FIG. 4 are formed.

Next, referring to FIGS. 30A and 30B, a thick silicon oxide layer 31 is deposited by a CVD process, and then, a CMP operation is performed on the silicon oxide layer 31. Thus, the surface of the silicon oxide layer 31 is flattenened. Then, an aluminum layer 32 is deposited on the silicon oxide layer 31 by a sputtering process.

Finally, referring to FIGS. 31A and 31B, the aluminum layer 32 is patterned by a photolithography and etching process, so that patterned aluminum layers 32a, i.e., the main bit lines MBL₁, MBL₂ and MBL₃ of FIG. 4 are obtained.

Thus, the contactless virtual ground type nonvolatile semiconductor device of FIG. 4 is also completed.

In FIG. 32, which is an equivalent circuit 35 showing a prior art contactless and type semiconductor memory device (see: JP-A-6-283721), main bit lines MBL₁, MBL₂, . . . are arranged along with sub bit lines SBL₁, SBL₂, . . . , Also, main source lines MSL₁, . . . are arranged along sub source lines SBL₁, . . . , For example, the main source line MSL₁ is arranged between the main bit lines MBL₁ and MBL₂, and the sub source line SBL₁ is arranged between the sub bit lines SBL₁, and SBL₂. Also, each of floating gate type nonvolatile memory cells M₁₁, M₁₂, . . . , M₂₁, M₂₂, . . . is connected to one of the word lines WL₁, WL₂, . . . , one of the sub bit lines SBL₁, SBL₂, . . . and one of the sub source lines SSL₁, SSL₂, . . . . The sub source lines SSL₁, . . . are connected to the main source lines MSL₁, . . . , respectively, via switching transistors Q₁₁, Q₁₁, . . . turned ON and OFF by the block selection signal BSEL₁.

In FIG. 33, which is a plan view illustrating a second embodiment of the present invention applied to the contactless and type nonvolatile semiconductor memory device of FIG. 32, grooves GV are provided between the sub bit line SBL₁ and the sub source line SSL₁ and between the sub source line SSL₁ and the sub bit line SBL₂. As a result, a distance between the sub bit line such as SBL₁ and the sub source line SSL₁ is increased substantially, and thus, leakage currents therebetween can be decreased, even if an erase voltage is increased. In addition, since insulating layers (not shown) are formed completely on the sub bit lines SBL₁, SBL₂, . . . and the sub source lines SBL₁, . . . , when patterning floating gate electrodes FG by a dry etching process, the sub bit lines SBL₁, SBL₂, . . . and the sub source lines SSL₁, . . . are hardly etched.

In FIG. 33, note that references WL₁, WL₂, . . . designate word lines, and STP designates a channel stopper, and F designates a field silicon oxide layer for isolating every two columns from each other.

The device of FIG. 33 is explained next in detail with reference to FIGS. 34A and 34B which are cross-sectional views taken along the lines I--I and II--II, respectively, of FIG. 33. In FIGS. 34A and 34B, the elements of FIGS. 17A and 17B are denoted by the same references. That is, field silicon oxide layers 41 of a rectangular shape, i.e., the field silicon oxide layers F are provided for every three N⁺ -type impurity layers 3a of a rectangular shape. Also, the polycrystalline silicon layers 7a (the floating gate electrodes FG) are wider than those of FIG. 17A, since no memory cell exists on the field silicon oxide layers 41(F).

The method of manufacturing the device of FIG. 33 is the same as that of manufacturing the device of FIG. 4 except for the field silicon oxide layers 41. For example, in the first manufacturing method as illustrated in FIGS. 5A, 5B through 17A and 17B, and in the second manufacturing method as illustrated in FIGS. 18A and 18B through 31A and 31B, field silicon oxide layers 41 are first grown on the silicon substrate 1 by a local oxidation of silicon (LOCOS) process, as illustrated in FIG. 35.

In FIG. 36, which is a plan view illustrating a third embodiment of the present invention, a contactless virtual ground type flash nonvolatile semiconductor device is illustrated. That is, erase gate electrodes ER₁, . . . are added to the elements of FIG. 4. Also, element isolation layers I are provided instead of the channel stoppers STP of FIG. 4. Even in FIG. 36, the grooves GV are provided between the sub bit lines SBL₁, SBL₂, SBL₃, . . . which are also formed by buried impurity layers. As a result, a distance between the sub bit lines SBL₁, SBL₂, SBL₃, . . . is increased substantially and thus, leakage currents therebetween can be decreased, even if an erase voltage is increased. Further, the channel length becomes larger than the distance between the sub bit lines SBL₁, SBL₂, SBL₃, . . . , which suppresses the short channel effect. In addition, when patterning floating gate electrodes FG by a dry etching process, the sub bit lines SBL₁, SBL₂, SBL₃, . . . are hardly etched.

In FIG. 36, note that references WL₁, WL₂, . . . designate word lines. Also, main bit lines MBL₁, MBL₂, MEL₃, . . . which are not shown are provided along the sub bit lines SBL₁, SBL₂, SBL₃, . . . . Further, AR designates an active area where isolation insulating layers are not formed.

A method for manufacturing the contactless virtual ground type flash nonvolatile semiconductor memory device of FIG. 36 is explained next with reference to FIGS. 37A, 37B, 38A, 38B, 39A, 39B, 40A, 40B, 41A, 41B, 42A, 42B, 43A, 43B, 44A, 44B, 45A, 45B, 46A, 46B, 47A, 47B, 48A, 48B, 49A, 49B, 50A, 50B, 51A, 51B, 52A, 52B, 53A and 53B. Note that FIGS. 37A through 53A are cross-sectional views taken along the line I--I of FIG. 36, and FIGS. 37B through 53B are cross-sectional views taken along the line II--II of FIG. 36.

First, referring to FIGS. 37A and 37B, an about 10 to 20 nm thick silicon oxide layer 42 is formed on a P⁻ -type silicon substrate 41 by using a thermal oxidation process or a CVD process. Then, about 1×10¹⁵ to 7×10¹⁵ arsenic ions cm² are implanted at an energy of about 50 keV into the silicon substrate 41, and an annealing operation is performed thereupon. Thus, an N⁺ -type impurity diffusion layer 43 is formed.

Next, referring to FIGS. 38A and 38B, the silicon oxide layer 42 is removed.

Next, referring to FIGS. 39A and 39B, an about 200 to 500 nm thick silicon oxide layer 44 is deposited on the N⁺ -type impurity diffusion layer 43.

Next, referring to FIGS. 40A and 40B, the silicon oxide layer 44 is patterned by a photolithography and etching process. As a result, patterned silicon oxide layers 44a of a rectangular shape are obtained. In this case, in FIG. 40B, the N⁺ -type impurity diffusion layer 43 is exposed.

Next, referring to FIGS. 41A and 41B, the N⁺ -type impurity diffusion layer 43 and the silicon substrate 41 are etched by a dry etching process using the silicon oxide layers 44a as a mask. As a result, patterned N⁺ -type impurity diffusion layers 43a are in self-alignment with patterned silicon oxide layers 44a. In this case, the N⁺ -type impurity diffusion layers 43a are of a rectangular shape and form the sub bit lines SBL₁, SBL₂ and SBL₃ of FIG. 36. Also, as a result, grooves 45 of a rectangular shape, i.e., the grooves GV of FIG. 36 are formed in self-alignment with the silicon oxide layers 44a. In this case, in FIG. 41B, the thickness of the silicon substrate 41 is reduced. Then, the silicon oxide layers 44a are removed.

Next, referring to FIGS. 42A and 42B, a thick silicon oxide layer 46 is deposited on the entire surface by a CVD process.

Next, referring to FIGS. 43A and 43B, the thick silicon oxide layer 46 is patterned by a photolithography and etching process, so that patterned silicon oxide layers 46a, i.e., the element isolation layers I of FIG. 36 are formed.

Next, referring to FIGS. 44A and 44B, a thermal oxidation operation is carried out, so that an about 7 to 10 nm thick gate silicon oxide layer 47 is formed. Then, a polycrystalline silicon layer 48 including phosphorus is deposited by a CVD process on the entire surface.

Next, referring to FIGS. 45A and 45B, the polycrystalline silicon layer 48 is patterned by a photolithography and etching process, so that polycrystalline silicon layers 48a of a rectangular shape are formed. Then, a gate insulating layer 49 made of ONO is formed on the entire surface.

Next, referring to FIGS. 46A and 46B, a polycrystalline silicon layer 50 including phosphorus is deposited on the gate insulating layer 49 by a CVD process. Then, a thick silicon oxide layer 51 is deposited on the entire surface by a CVD process.

Next, referring to FIGS. 47A and 4713, the silicon oxide layer 51 is patterned by a photolithography and etching process, so that patterned silicon oxide layers 51a are obtained. Then, the polycrystalline silicon layer 50 is patterned by an etching process using the silicon oxide layers 51a as a mask. As a result, patterned polycrystalline silicon layers 50a, i.e., the word lines WL₁, WL₂, WL₃ and WL₄ of FIG. 36 are obtained.

Next, referring to FIGS. 48A and 48B, a silicon oxide layer (not shown) is deposited on the entire surface, and the silicon oxide layer is etched back by an anisotropic (dry) etching process. As a result, sidewall silicon oxide layers 52 are formed on sidewalls of the silicon oxide layers 51a and the polycrystalline silicon layers 50a.

Next, referring to FIGS. 49A and 49B, the polycrystalline silicon layers 48a are patterned by an RIE process using the silicon oxide layers 51a and the sidewall silicon oxide layers 52 as a mask. In this case, the silicon oxide layers 46a serve as an etching stopper. Thus, patterned polycrystalline silicon layers 48b, i.e., the floating gate electrodes FG of FIG. 36 are obtained.

Next, referring to FIGS. 50A and 50B, a thermal oxidation is carried out, so that an about 10 nm thick silicon oxide layer 53 is grown on the sidewalls of the polycrystalline layers 48b (the floating gate electrodes FG).

Next, referring to FIGS. 51A and 51B, a polycrystalline silicon layer 53 is deposited and patterned, so that the erase gate electrodes ER₁, . . . of FIG. 36 are obtained.

Next, referring to FIGS. 52A and 52B, a thick silicon oxide layer 54 is deposited by a CVD process, and then, a CMP operation is performed on the silicon oxide layer 54. Thus, the surface of the silicon oxide layer 54 is flattened. Also, contact holes (not shown) for the sub bit lines SBL₁, SBL₂, SBL₃, . . . are perforated in the layers 54 and 51a. Then, an aluminum layer 55 is deposited on the silicon oxide layer 54 by a sputtering process.

Finally, referring to FIGS. 53A and 53B, the aluminum layer 55 is patterned by a photolithography and etching process, so that patterned aluminum layers 55a, i.e., the main bit lines MBL₁, MBL₂ and MBL₃ of FIG. 36 are obtained.

Thus, the contactless virtual ground type flash nonvolatile semiconductor device of FIG. 36 is completed.

As explained hereinabove, according to the present invention, since grooves are provided between buried impurity diffusion layers for sub bit lines and sub source lines, even i;f the integration is advanced so that the spacing between the buried impurity diffusion layers becomes narrow, leakage current flowing therebetween can be suppressed. In addition, since insulating layers on the buried impurity diffusion layers are sufficiently thick, when patterning floating gate electrodes, the buried impurity diffusion layers are hardly etched, which can suppress the reduction of the read operation speed. Further, since the grooves serving as channel regions are in self-alignment with the buried impurity diffusion layers, the fluctuation of the operation characteristics of nonvolatile memory cells can be suppressed. 

What is claimed is:
 1. A contactless nonvolatile semiconductor memory device of a virtual ground type, comprising:a semiconductor substrate; a plurality of sub bit lines of the virtual ground type memory, each of said sub bit lines being an impurity diffusion layer in said substrate that has a rectangular shape in plan view with a longer side along a first direction; a plurality of grooves of a rectangular shape in said semiconductor substrate, each of said grooves being between two of said sub bit lines; a first gate insulating layer on said semiconductor substrate at said grooves; a plurality of floating gate electrodes on said first gate insulating layer; a second gate insulating layer on said floating gate electrodes; and a plurality of word lines on said second gate insulating layer, said word lines being along a second direction perpendicular to said first direction, each of said sub bit lines crossing beneath plural said word lines.
 2. The device as set forth in claim 1, further comprising a plurality of insulating layers of a rectangular shape, each on one of said sub bit lines.
 3. The device as set forth in claim 2, wherein said insulating layers are thicker than said second gate insulating layer.
 4. The device as set forth in claim 1, further comprising a plurality of channel stoppers in said semiconductor substrate on bottoms of said grooves.
 5. The device as set forth in claim 1, further comprising:an insulating layer on said word lines; and a plurality of main bit lines on said insulating layer, each of said main bit lines extending over one of said sub bit lines and being connected to said one of said sub bit lines.
 6. The device as set forth in claim 1, further comprising:a first insulating layer on said word lines; a second insulating layer on sidewalls of said floating gates; a plurality of erase gate electrodes on said first and second insulating layers, said erase gate electrodes being in parallel with said word lines.
 7. A contactless nonvolatile semiconductor memory device of an AND type, comprising:a semiconductor substrate; a first sub bit line, a sub source line, and a second sub bit line that are, respectively, first, seconds and third impurity diffusion layers in said substrate, each having a rectangular shape in plan view with a longer side along a first direction; a plurality of grooves of a rectangular shape in said semiconductor substrate, each of said grooves being between two of said impurity diffusion layers; first and second field insulating layers of a rectangular shape that are on an outer side of said first sub bit line and an outer side of said second sub bit line, respectively; a first gate insulating layer on said semiconductor substrate within said grooves; a plurality of floating gate electrodes on said first insulating layer; a second gate insulating layer on said floating gate electrodes; and a plurality of word lines on said second gate insulating layer, said word lines being along a second direction perpendicular to said first direction, each of said sub bit lines crossing beneath plural said word lines.
 8. The device as set forth in claim 7, further comprising a plurality of insulating layers of a rectangular shape each on one of said impurity diffusion layers.
 9. The device as set forth in claim 8, wherein said insulating layers are thicker than said second gate insulating layer.
 10. The device as set forth in claim 7, further comprising:an insulating layer on said word lines; and a first main bit line, a main source line and a second main bit line on said insulating layer, said first main bit line, said main source line and said second main bit line extending over said first sub bit line, said sub source line and said second main bit line, respectively. 